Huntington's disease (HD) belongs to a family of at least eight inherited, untreatable neurodegenerative diseases. Each is caused by an abnormal polyglutamine (polyQ) expansion in a different protein. In each, disease occurs when the polyQ stretch exceeds a certain length and symptom onset is inversely related to its length. Abnormal deposits of protein called inclusion bodies (IBs) characterize many of these diseases. Whether IBs are pathogenic, an epiphenomenon, or a beneficial defense response is controversial. Abnormal polyQ expansions probably cause degeneration by conferring a toxic gain of function to proteins. PolyQ expansions may adopt a conformation that is different depending on whether huntingtin (htt) is in IBs or not. We hypothesize that abnormal polyQ expansion alters the conformation of soluble mutant htt, enabling it to interact with cellular targets and produce neurodegeneration, independent of IB formation. Certain heat shock proteins (HSPs) can protect cells against polyQ and regulate IB formation. It is unknown whether they act mainly on IBs, oligomers, or malfolded monomers. It is unknown whether htt needs to oligomerize to adopt a toxic conformation and whether the length or the composition of the polyQ stretch is critical to conformation or aggregation. We have used primary neurons to develop a model of HD that recapitulates polyQ-dependent and neuron-specific death and IB formation. We have also developed monoclonal antibodies that bind a conformation of htt that correlates closely with HD symptoms and which distinguishes htt in IBs from more soluble forms. Finally, we have built a robotic microscope so we can simultaneously measure IB formation within living neurons and then track their individual fates. We propose to use these tools to accomplish the following Specific Aims: Aim 1. To determine whether the availability of a disease-associated conformation is a better predictor of neurodegeneration than IB formation. Aim 2. To determine whether heat shock proteins regulate htt conformation and to relate the effects of heat shock proteins on neuronal survival to the effects on conformation and on inclusion body formation. Aim 3. To determine whether htt must oligomerize or contain a contiguous stretch of pure glutamine residues to form a disease-associated conformation.